Catalysts for nox reduction employing h2 and a method of reducing nox

ABSTRACT

Disclosed is a catalyst composition for reducing NOx through two steps including reacting NOx with H2 thus producing ammonia which is then reacted with NOx, instead of direct NOx reduction by H2, and a method of reducing NOx using the catalyst composition.

TECHNICAL FIELD

As a system for reducing NO_(x) from diesel exhaust gas, selective catalytic reduction (SCR) and NO_(x) storage reduction (NSR) are known. In the SCR method, a reducing agent is continuously sprayed into exhaust gas and selectively reacted with NO_(x) in a catalyst bed so that NO_(x) is converted into N₂. This method is classified into NH₃-SCR, Urea-SCR, HC-SCR and H₂-SCR depending on the type of reducing agent such as ammonia, urea, hydrocarbon and H₂. On the other hand, in the NSR method, NO_(x) is stored in an oxidation atmosphere and then desorbed in a reduction atmosphere formed through spray of fuel, thus reducing NO_(x).

BACKGROUND ART

Generally, NO_(x) emitted from large-scale boilers or nitric acid plants can be effectively removed through NH₃-SCR which supplies ammonia as a reducing agent to a catalyst bed composed of titania-supported vanadia or iron-containing zeolite. Ammonia is highly reactive and selective and is thus very effective for removing NO_(x) from the exhaust gas of fixed facilities even in the presence of O₂. However, the use of ammonia to remove NO_(x) from diesel exhaust gas is very dangerous because a diesel vehicle should be driven in a state of always being loaded with ammonia which is highly toxic. So, aqueous urea is used instead of ammonia as a reducing agent therein. The urea is decomposed into ammonia and carbon dioxide in the catalyst bed so that NO_(x) is reduced to N₂. Although the urea-SCR method is advantageous because NO_(x) removal performance is high, it is problematic in that a tank for storing aqueous urea and a device for spraying such urea should be additionally mounted to a diesel vehicle. As has been done for fuel, a sales network of aqueous urea should be constructed. As well, the urea-SCR method is difficult to apply to a diesel vehicle, due to problems including low solubility of urea, freezing, and ammonia slip.

Among the SCR methods, H₂-SCR using H₂ as a reducing agent instead of the aqueous urea is receiving attention because the construction of an apparatus thereof is simple and there is no concern about secondary pollution. However, it is difficult to construct the supply network of H₂ and to load it into a vehicle. Further, O₂ in the diesel exhaust gas may first react with H₂, undesirably lowering NO_(x) selective removal efficiency by H₂. Thus, the application of the above method has not been considered to date. The reason is described below.

The temperature and O₂ content of diesel exhaust gas greatly vary depending on driving conditions of vehicles.

During normal high-speed driving, the temperature may be 300° C. or higher and also the O₂ content may exceed 10% under lean burn. Further, in order to allow NO₂ to be directly reduced to N₂ by H₂, H₂ should be strongly activated. In this case, however, a probability of reacting such H₂ with O₂ is increased, undesirably lowering the NO_(x) removal efficiency. Namely, to increase the NO_(x) removal efficiency by H₂, the probability of reacting H₂ with O₂ should be inhibited while increasing the degree of activation of H₂, which is difficult. Hence, limitations are imposed on applying the H₂-SCR method to diesel vehicles.

DISCLOSURE Technical Problem

Accordingly, the present inventors have directed their attention to a method of reducing NO_(x) using H₂ as a reducing agent in the presence of ammonia, in lieu of conventional direct NO_(x) reduction by H₂, to selectively reduce NO_(x) while inhibiting excessive activation of H₂. The present inventors have devised two-step NO₂ removal, including activating H₂ only to the appropriate level so that it can thus react with NO_(x), giving ammonia, which is then reacted with NO_(x).

Therefore, an object of the present invention is to provide a catalyst composition for reducing NO_(x) through two steps including reacting NO_(x) with H₂, thus preparing ammonia, which is then reacted with NO_(x), thereby removing NO_(x), instead of direct NO_(x) reduction by H₂.

Another object of the present invention is to provide a hybrid catalyst composition having not only a function as an SCR catalyst of reaction between a reducing agent and NO_(x) but also an NSR function for storing NO_(x) on the surface of the catalyst to thus react with the reducing agent, so that part of the reducing agent is adsorbed on the surface of the catalyst and thus NO_(x) storage sites are formed, in order to efficiently remove NO_(x) regardless of changes in the concentration of NO_(x).

A further object of the present invention is to provide a catalyst composition suitable for a H₂-SCR method using H₂ as a reducing agent including the two steps of producing ammonia and then removing NO_(x), in which the temperature range of the catalyst composition usable in diesel vehicles is wide and is on the order of 150˜300° C.

Still another object of the present invention is to provide a catalyst composition including the catalyst composition according to the present invention and a conventional NH₃-SCR catalyst composition, which are mixed together.

Yet another object of the present invention is to provide a method of reducing NO_(x) using the catalyst composition.

Technical Solution

In order to accomplish the above objects, the present invention provides a mixed oxide catalyst, a method of preparing the catalyst and a method of reducing NO_(x) using the catalyst.

According to the present invention, the mixed oxide catalyst for reducing NO_(x) using H₂ as a reducing agent includes one or more selected from the group of A metal oxides consisting of Fe₂O₃, CO₂O₃, NiO and CuO, and one or more selected from the group of B metal oxides consisting of V₂O₅, Cr₂O₃, MnO₂ and MoO₃, which are co-precipitated and mixed. The present invention has the following features, but is not limited thereto.

In the mixed oxide catalyst, the weight ratio of A metal oxide to B metal oxide may range from 2:1 to 1:0.5.

On the mixed oxide catalyst, a precious metal selected from the group consisting of Pt and Pd may be supported in an amount of 0.1˜2 wt %.

The mixed oxide catalyst may further include a conventional NH₃-SCR catalyst. As such, an example of the conventional NH₃-SCR catalyst may include, but is not limited to, a titania-supported vanadia catalyst or an iron-containing zeolite catalyst.

In addition, the method of preparing the mixed oxide catalyst for reducing NO_(x) using H₂ as a reducing agent includes dissolving in aqueous nitrate or acetate one or more metal oxide precursors selected from the group of A metal oxides consisting of Fe₂O₃, Co₂O₃, NiO and CuO, and one or more metal oxide precursors selected from the group of B metal oxides consisting of V₂O₅, Cr₂O₃, MnO₂ and MoO₃, thus obtaining a solution, adding barium nitrate for improving structural stability of the mixed oxide to the solution, adding ammonia water or aqueous sodium bicarbonate as a precipitating agent to the solution so that pH of the solution is 7˜7.3, thus forming a precipitate, and subjecting the precipitate to post treatment including filtering, washing, drying and burning. The present invention has the following features, but is not limited thereto.

In the method, the ratio of A metal oxide precursor to B metal oxide precursor may range from 2:1 to 1:0.5.

The method may further include, after subjecting the precipitate to post treatment, supporting either or both precious metals of Pt and Pd in an amount of 0.1˜2 wt % on the catalyst, burning the precious metal-supported catalyst, and subjecting the burned catalyst to reduction treatment using a gas mixture containing N₂ and H₂ at a molar ratio of 1.

In addition, the method of reducing NO_(x) includes reducing NO_(x) at 150˜350° C. using H₂ as a reducing agent in the presence of the above mixed oxide catalyst. This method may be performed even under conditions in which O₂ content is 0˜10% by volume based on the volume of NO_(x), but the present invention is not limited thereto.

Advantageous Effects

According to the present invention, oxides of A metals including Cu, Fe, Co and Ni and oxides of B metals including Cr, Mn, Mo and V are co-precipitated and mixed, thus preparing mixed oxide catalysts and ternary mixed oxide catalysts. These catalysts exhibit superior activity for production of ammonia through selective reaction between NO₂ and H₂ even in the presence of 5% or 10% O₂, and simultaneously, manifest very high NO₂ and NO storage performance. Thereby, the catalysts can exhibit superior NO_(x) reduction performance through injection of H₂ even in the presence of O₂. In particular, because NO_(x) is removed via ammonia, the NO_(x) removal performance is high. The catalysts according to the present invention have high Pt or Pd dispersability and high hydrothermal stability and poisoning resistance to sulfur and thus can significantly remove NO_(x) from diesel exhaust gas. Also, the catalysts can be typically easily prepared from transition metal precursors which are inexpensive with high durability to water or heat and to sulfur poisoning.

DESCRIPTION OF DRAWINGS

FIG. 1 shows X-ray diffraction patterns of A-B mixed oxide catalysts in which the B metal is Cr and the A metal is Fe, Co, Ni and Cu, after a burning process;

FIG. 2 shows X-ray diffraction patterns of A-B mixed oxide catalysts in which the A metal is Cu and the B metal is Cr, V, Mo and Mn, a Cr—Mn catalyst, and a Fe—Mn catalyst, after a burning process;

FIG. 3 shows X-ray diffraction patterns of Pt(2.0)-Cu—Cr and Pt(2.0)-Fe—Mn catalysts, which are Pt-supported catalysts;

FIG. 4 shows IR spectra of the process of storing NO₂ (a) and the process of reducing NO₂ by H₂ (b) in the Cu—Cr catalyst;

FIG. 5 shows IR spectra of the process of producing ammonia from NO₂ through injection of H₂ in the absence of O₂ (a) and in the presence of O₂ (b) in the Cu—Cr catalyst which is a mixed oxide catalyst;

FIG. 6 shows IR spectra of the Cu—Mn catalyst, the Fe—Cr catalyst and the Fe—Mn catalyst in the presence of O₂;

FIG. 7 schematically shows a flow reactor used for H₂-SCR;

FIG. 8 shows NO₂ reduction results by H₂ of the Pt(2.0)-Fe catalyst (a), the Pt(2.0)-Mn catalyst (b) and the Pt(2.0)-Fe—Mn catalyst (c) under flow of 500 ppm NO₂ containing 10% O₂; and

FIG. 9 shows NO₂ reduction behavior by H₂ of the Pt(2.0)-Fe—Mn catalyst with Fe-BEA zeolite.

MODE FOR INVENTION

A catalyst for effectively and selectively reducing NO_(x) using H₂ gas in the presence of O₂ should have the following three functions, namely, high NO_(x) adsorption, appropriate activation of H₂, and activation of adsorbed NO_(x). Specifically, NO_(x) is adsorbed or stored on the surface of the catalyst because of having reacted therewith and thus should be concentrated on the catalyst. In this case, part of NO_(x) should be activated so that it is converted into ammonia through reaction with H₂. As well, when H₂ is adsorbed on the surface of the catalyst and thus activated in an atomic state, it may be reacted with NO_(x), thus producing ammonia.

Taking into consideration the above reasons, the present inventors selected, as oxides of A metals which exhibit superior NO_(x) storage performance, Fe₂O₃, CO₂O₃, NiO and CuO, and as oxides of B metals which are able to adsorb NO_(x) in an activated state, V₂O₅, Cr₂O₃, MnO₂ and MoO₃. In the present invention, the oxides of the A and B metals are co-precipitated and combined, thus preparing mixed oxides, thereby maximizing NO_(x) storage capacity and NO_(x) reduction performance at the same time. Further, to inhibit the sintering of the B metal, barium nitrate is added as a structure stabilizer. As a precious metal, Pt or Pd is supported on the catalyst. Depending on the type of catalyst, two or more kinds of A metal or B metal are added, thus preparing ternary or more mixed oxide catalysts, after which a precious metal is supported thereon. Accordingly, H₂ is activated in an atomic state on the surface of precious metal to thus react with NO_(x) adsorbed on the surface of the mixed oxide, thereby producing ammonia. In the present invention, the mixed oxide according to the present invention promotes the ammonia and NO_(x) reduction and ultimately reduces NO_(x) to N₂.

For mass production of the mixed oxide catalyst at low cost, a general co-precipitation method is applied, and inexpensive starting materials are used. When ammonia water or sodium bicarbonate is added to the mixed solution to appropriately adjust the pH of the solution, a highly active catalyst is prepared. The precious metal is supported in an amount of 0.1˜2%, and the reduction reaction is operated in the temperature range of 150˜350° C. Even when the concentration of O₂ in NO_(x) exceeds 10% by volume, the above catalyst can exhibit superior NO_(x) selective reduction by H₂.

A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

Example 1 Preparation of Mixed Oxide Catalysts

Mixed oxide catalysts were prepared from oxides of A metals (Cu, Fe, Co, Ni) and B metals (Cr, Mn, Mo, V) through co-precipitation and mixing. The weight ratio of A metal oxide to B metal oxide was adjusted to 2, 1 and 0.5. As a precipitating agent, ammonia water or aqueous sodium bicarbonate was used, and pH of the mixed solution was adjusted to 6.0˜8.0. Any one A metal was reacted with any one B metal, thus preparing binary mixed oxide catalysts, and also, multicomponent mixed oxide catalysts were prepared using two or more kinds of these metals. The method of preparing some catalysts which are regarded as important is described below.

a) Cu—Cr Mixed Oxide Catalyst

A solution of 15.2 g of copper nitrate and 1.60 g of barium nitrate in 152 g of water was mixed with a solution of 15.4 g of potassium dichromate in 154 g of water, thus preparing a mixed solution. For sufficient mixing, the mixed solution was stirred for 30 min and then ammonia water was slowly added thereto so that the pH thereof was 7.0˜7.5. The resultant precipitate was filtered using filter paper, sufficiently dried in an oven at 80° C., and then ground using a mortar and a pestle, thus obtaining fine powder. The powder was transferred into an electric furnace so that it was burned at 500° C. for 4 hours, giving 10.9 g of a Cu—Cr mixed oxide catalyst represented by a Cu—Cr catalyst.

b) Fe—Cr Mixed Oxide Catalyst

A solution of 24.7 g of iron nitrate and 1.6 g of barium nitrate in 247 g of water and a solution of 15.4 g of potassium dichromate in 154 g of water were prepared. These two solutions were mixed and stirred for 30 min to provide for sufficient mixing, after which ammonia water was slowly added thereto so that the pH thereof was 7.0˜8.0. The resultant precipitate was filtered using filter paper, dried in an oven at 80° C., and then ground using a mortar and a pestle, thus obtaining fine powder. The powder was transferred into an electric furnace so that it was burned at 500° C. for 4 hours, giving 8.5 g of a Fe—Cr mixed oxide catalyst represented by a Fe—Cr catalyst.

c) Cu—Mn Mixed Oxide Catalyst

A solution of 20.6 g of copper nitrate in 206 g of water and a solution of 24.6 g of manganese nitrate in 246 g of water were prepared. These two solutions were sufficiently mixed for 30 min, after which a 1 M sodium bicarbonate solution was slowly added thereto so that the pH thereof was 7.0˜7.8. The resultant precipitate was filtered using filter paper, dried in an oven at 80° C., and then ground using a mortar and a pestle, thus obtaining fine powder. The powder was transferred into an electric furnace so that it was burned at 500° C. for 4 hours, giving 21 g of a Cu—Mn mixed oxide catalyst represented by a Cu—Mn catalyst.

d) Fe—Mn Mixed Oxide Catalyst

A solution of 35.9 g of iron nitrate in 359 g of water and a solution of 25.5 g of manganese nitrate in 255 g of water were prepared. These two solutions were sufficiently mixed for 30 min, after which a 1 M sodium bicarbonate solution was slowly added thereto so that the pH thereof was 6.5˜7.5. The resultant precipitate was filtered using filter paper, dried in an oven at 80° C., and then ground using a mortar and a pestle, thus obtaining fine powder. The powder was transferred into an electric furnace so that it was burned at 500° C. for 4 hours, giving 15 g of a Fe—Mn mixed oxide catalyst represented by a Fe—Mn catalyst.

e) Mn—Cr Mixed Oxide Catalyst

A solution of 17.6 g of manganese nitrate and 1.6 g of barium nitrate in 176 g of water and a solution of 15.4 g of potassium dichromate in 154 g of water were prepared. These two solutions were mixed and stirred for 30 min to provide for sufficient mixing, after which ammonia water was slowly added thereto so that the pH thereof was 7.0˜7.5. The resultant precipitate was filtered using filter paper, dried in an oven at 80° C., and then ground using a mortar and a pestle, thus obtaining fine powder. The powder was transferred into an electric furnace so that it was burned at 500° C. for 4 hours, giving 12.1 g of a Mn—Cr mixed oxide catalyst represented by a Mn—Cr catalyst.

The other mixed oxide catalysts were prepared through the above procedures. Also, mixed oxide catalysts having a composition ratio of 2 and 0.5, in addition to 1, were prepared. Each of the catalysts thus prepared was reduced under flow of a reducing gas mixture containing H₂ and N₂ at a molar ratio of 1:1 at 400° C. and a flow rate of 120 ml/min, before being used.

f) Synthesis of Fe-BEA Zeolite (NH₃-SCR Catalyst)

To a solution of 377 g of tetraethyl ammonium hydroxide and 312 g of colloidal silica, 31 g of sodium aluminate was dropped with stirring, thus preparing a synthesis mother solution having a composition of 1.5Na₂O:20SiO₂:Al₂O₃: 2.46(TEA)₂O:416H₂O. This solution was stirred for 2 hours, placed in a high-pressure autoclave and thus heated to 165° C. for 3 hours, and then subjected to hydrothermal reaction while maintaining the above temperature for 90 hours, thus obtaining 28 g of BEA zeolite having a molar ratio of Si/Al of 10. 10 g of the synthesized BEA zeolite was added to 100 ml of a 0.2 N iron chloride solution, ion-exchanged at 60° C. for two days, filtered, washed, and then burned, giving Fe-ion-exchanged Fe-BEA zeolite.

Example 2 Preparation of Pt- or Pd-supported Mixed Oxide Catalyst

To evaluate NO₂ reduction performance by a H₂ reducing agent, Pt was supported in an amount of 0.1, 0.2, 1.0 and 2.0% by weight on the mixed oxide catalyst of Example 1. As a Pt precursor, hexachloroplatinic acid was dissolved in an amount of each of 0.1, 0.2, 1.1 and 2.1 g in 35 g of water, thus preparing a Pt solution, which was then added to 50 g of the mixed oxide catalyst. The catalyst reached equilibrium after 24 hours, and then dried in an oven at 80° C. and thus dewatered. The solution was burned in an electric furnace at 400° C. for 2 hours, placed in a quartz tube and then subjected to reduction treatment using a gas mixture containing N₂ and H₂ mixed at an equal ratio. The Pt-supported Cu—Cr catalysts and Fe—Mn catalysts were represented by Pt(0.1)-Cu—Cr, Pt(0.2)-Cu—Cr, Pt(1.0)-Cu—Cr, Pt(2.0)-Cu—Cr, Pt(0.1)-Fe—Mn, Pt(0.2)-Fe—Mn, Pt(1.0)-Fe—Mn, and Pt(2.0)-Fe—Mn.

On the other hand, Pd-supported mixed oxide catalysts were prepared using a palladium nitrate precursor through procedures similar to the above Pt supporting procedures. Specifically, palladium nitrate was dissolved in an amount of each of 0.1, 0.2, 1.1 and 2.2 g in 35 g of water, thus preparing a Pd solution which was then added to 50 g of the mixed oxide catalyst, dried, burned in an electric furnace at 400° C. for 2 hours, and then subjected to reduction treatment, yielding Pd-supported mixed oxide catalysts. The Pd-supported Cu—Cr catalysts and Fe—Mn catalysts were represented by Pd(0.1)-Cu—Cr, Pd(0.2)-Cu—Cr, Pd(1.0)-Cu—Cr, Pd(2.0)-Cu—Cr, Pd(0.1)-Fe—Mn, Pd(0.2)-Fe—Mn, Pd(1.0)-Fe—Mn, and Pd(2.0)-Fe—Mn.

Example 3 X-ray Diffraction Pattern of A-B Mixed Oxide Catalyst

Among the A-B mixed oxide catalysts prepared in Example 1, the catalysts in which the B metal was Cr and the A metal was Fe, Co, Ni and Cu were burned, after which X-ray diffraction patterns thereof were measured. The results are shown in FIG. 1. The diffraction pattern of the mixed oxide catalyst was very complicated because the diffraction peaks of metal oxides alone and in combinations thereof coexisted. In the Cu—Cr catalyst, the diffraction peaks of CuO, CuCr₂O₄ and BaCrO₄ were shown. In any catalyst, the diffraction peak of BaCrO₄ added to improve structural stability of the catalyst was distinctly observed. In the Cu—Cr and Fe—Cr catalysts, the diffraction peak of CuO or Fe₂O₃ was strongly observed. In the Fe—Cr and Co—Cr catalysts, the diffraction peaks difficult to confirm were present, and thus the mixed oxide catalysts were seen to have a complicated structure.

FIG. 2 shows the X-ray diffraction patterns of the A-B mixed oxide catalysts in which the A metal was set and the kind of B metal was changed to Cr, V, Mo and Mn. In the Cu—V catalyst, the diffraction peaks of Cu₂V₂O₇ and V₂O₅ were observed. In the Cu—Mn catalyst, the peak of CuO and CuMnO₄ was observed together. Whereas, in the Cu—Mo catalyst, the diffraction peak of the copper compound was unclear, and only the diffraction peaks of MoO₂ and MoO₃ were greatly observed. The diffraction peaks of the Cr—Mn catalyst prepared from only B metals and the Fe—Mn catalyst prepared through crossing of A-B metals were observed. Depending on the kind of metal, the diffraction pattern of the catalyst was seen to considerably vary. In the Cu—Mn catalyst, the diffraction peaks of CuO and CuMnO₄ were observed, whereas in the Cr—Mn catalyst the diffraction peaks of MnO₂ and BaCrO₄ were observed. Also, in the Fe—Mn catalyst, the diffraction peaks of Fe₂O₃ and NaMnO₄ were observed. From this, the oxides of A and B metals could be seen to be present in different forms depending on the kind of metal.

FIG. 3 shows X-ray diffraction patterns of, as Pt-supported catalysts, Pt(2.0)-Fe, Pt(2.0)-Mn, Pt(2.0)-Fe—Mn, Pt(2.0)-Fe—Mn(2/1) and Pt(2.0)-Fe—Mn(1/2) having different Fe/Mn composition ratios, and Pt(2.0)-Cu—Cr catalysts, and as a Pd-supported catalyst, Pd(2.0)-Fe—Mn, which were prepared in Example 2. In the Pt(2.0)-Fe catalyst, the diffraction peaks of NaFeO₂ and Fe metal were observed. In the Pt(2.0)-Mn catalyst, the diffraction peaks of MnO were greatly observed. In the Pt(2.0)-Fe—Mn, Pt(2.0)-Fe—Mn(2/1) and Pt(2.0)-Fe—Mn(1/2) catalysts containing Fe and Mn, not only the diffraction peaks of MnO and Fe metal but also the diffraction peak of MnFe₂O₄ were observed, from which it could be seen that Fe and Mn were not present alone. The diffraction peak of the Pd-supported catalyst, namely, the Pd(2.0)-Fe—Mn catalyst was similar to that of the Pt-supported catalyst, namely, Pt(2.0)-Fe—Mn catalyst, with the exception that the diffraction peaks of NaFeO₂ were weakly observed. The Pt(2.0)-Cu—Cr catalyst had complicated diffraction peaks, unlike the Cu—Cr catalyst. The diffraction peak of Pt in all of these catalysts was not observed. This was judged to be because Pt did not aggregate on the surface of the catalyst but was well dispersed thereon.

Example 4 NO₂ Storage Performance of Mixed Oxide Catalyst

The NO₂ storage performance of the catalysts prepared in Examples 1 and 2 was evaluated. To this end, the catalyst was loaded into a gravimetric adsorption system provided with a quartz spring and then exhausted at 300° C. for 1 hour, after which measurement was performed at 150° C. in consideration of the temperature of diesel exhaust gas. The results of measurement of the storage performance of the Cu—Cr catalyst and the Fe—Mn catalyst among the above mixed oxide catalysts are summarized in Table 1 below. The NO₂ storage performance of the catalyst was greatly changed depending on catalyst pretreatment conditions. The storage performance was represented into a storage amount in a state where the catalyst was exposed to NO₂ at 30 Torr and a storage amount in a state where NO₂ was emitted. Before reduction treatment, an adsorption amount was slightly larger than the storage amount. However, after the reduction treatment, the adsorption amount became similar to the storage amount. This is considered to be because part of NO₂ is weakly adsorbed on the surface of the catalyst before the reduction treatment, but the entirety thereof is strongly stored thereon after the reduction treatment. Thus, the performance of the catalyst was determined only by the storage amount with no consideration being given to the adsorption amount. In the case of the Fe catalyst prepared from only Fe, the amount of stored NO₂ was increased about 23 times from 7 mg/g to 161 mg/g through reduction treatment. Also, in the case of the Mn catalyst prepared from only Mn, the amount of stored NO₂ was increased about 6 times from 13 mg/g to 79 mg/g through the reduction treatment. The Fe—Mn mixed oxide catalyst containing these two components increased the storage amount about 7 times before the reduction treatment but about 2 times after the reduction treatment, compared to that of the catalyst composed exclusively of Fe or Mn. In the case of the Fe—Mn catalyst, the amount of stored NO₂ was remarkably larger than that of the Cu—Cr catalyst, and thus the NO₂ storage performance could be seen to greatly vary depending on the kind of metal. As such, although the storage amount may vary depending on the kind of metal and the reduction treatment, NO₂ may be typically adsorbed in a large amount on many O vacancies formed on the surface of the catalyst after the reduction treatment. After the reduction treatment, the amount of NO₂ stored on the Fe—Mn catalyst was 174 mg/g, which was evaluated to be superior.

Table 1 below shows the NO₂ adsorption and storage amounts of the mixed oxide catalysts at 50° C.

TABLE 1 NO₂ Adsorption and Storage Amounts (mg/g) Before Reduction After Reduction Adsorption Storage Adsorption Storage Catalyst Amount Amount Amount Amount Fe 15  7 161 161 Mn 24 13  79  79 Fe—Mn 90 83 174 174 Cu—Cr 22 15  50  43 Pt (2.0)—Fe — — 254 254 Pt (2.0)—Mn — — 106 106 Pt (2.0)—Fe—Mn — — 182 181 Pt (2.0)—Cu—Cr — —  39  36

When Pt was supported on the metal oxide catalyst, the amount of stored NO₂ was greatly changed. After reduction treatment of the precious metal-supported catalyst, namely, the Pt(2.0)-Fe catalyst, the amount of stored NO₂ was 254 mg/g, which was esteemed to be very high. The Pt(2.0)-Mn catalyst also increased the storage amount from 79 mg/g to 106 mg/g, which was smaller than that of the Pt(2.0)-Fe catalyst. The Pt(2.0)-Fe—Mn catalyst slightly increased the storage amount from 170 mg/g to 181 mg/g, compared to that of the Fe—Mn catalyst containing no Pt. However, the Cu—Cr catalyst slightly decreased the storage amount from 43 mg/g to 36 mg/g when Pt was supported thereon. This is considered to be because, when Pt is supported, the activation of H₂ is increased upon reduction treatment and thus many O vacancies of the surface of the catalyst are formed. However, in the case of the Pt(2.0)-Cu—Cr catalyst, the amount of stored NO₂ was decreased, and thus the storage performance of the catalyst could be seen to greatly vary depending on the kind of metal. The storage state of the adsorbed NO was checked using an IR spectrometer (BIO-RAD, 175C) equipped with an in-situ cell. The NO₂ storage behavior and the NO₂ desorption behavior by H₂ in the Pt(2.0)-Cu—Cr catalyst of Example 1 are shown in FIG. 4. The reduction treatment was performed at 250° C. using H₂ at a flow rate of 100 ml/min, after which 2000 ppm NO₂ by volume was allowed to flow at 200° C. and thus the process of storing NO₂ was measured (a). Also, while N₂ gas containing 20% H₂ by volume was allowed to flow, the process of desorbing the stored NO₂ was measured (b). When NO₂ was stored, initial absorption bands were shown at 1540, 1420 and 1240 cm⁻¹. After 10 min, the absorption bands were greatly increased at 1440 and 1340 cm⁻¹. Initially, NO₂ was stored in the form of bidentate nitrate and ionic nitrite, and then was converted into ionic nitrate over time. When H₂ was allowed to flow to the Pt(2.0)-Cu—Cr catalyst to which NO₂ was stored, the absorption bands were rapidly decreased and then almost none thereof was seen after 15 min. This was because NO₂ stored on the catalyst by H₂ was rapidly reduced and desorbed.

Example 5 Production of Ammonia in Mixed Oxide Catalyst

In the mixed oxide catalyst, ammonia was produced through reaction between H₂ and NO₂. The produced ammonia was strongly adsorbed on acid sites and could thus be detected using an IR spectrometer used in Example 4. A mesoporous material (MCM-41) having a sulfonic acid group able to strongly adsorb ammonia was used as a test catalyst. The test catalyst was fixed to the path through which IR beams were passed. While the catalyst was heated to 250° C. using a heater, H₂ was added at 30 Torr and thus reduction treatment was performed for 1 hour. After exhaust, NO₂ was fed at 20 Torr at the same temperature. Because NO₂ was not stored on acid sites, there was no difference in the test sample. Subsequently, H₂ was added at 20 Torr so that the reaction was performed for 20 min and cooling to 50° C. was performed, thus checking whether ammonia was produced.

To evaluate the effect of O₂, O₂ was added at 20 Torr after NO₂ adsorption. FIG. 5 shows the results of the production of ammonia by adding H₂ to the Cu—Cr mixed oxide catalyst of Example 1 to which NO₂ was adsorbed, in the absence of O₂ (a) and in the presence of O₂ (b). In the absence of O₂, reduction treatment was performed at 250° C., after which the absorption band of sulfonic acid was greatly observed at 1377 cm⁻¹. However, when H₂ was injected and reaction and then cooling to 50° C. were performed, the absorption band at 1377 cm⁻¹ became small and novel absorption bands were shown at 1440 and 1410 cm⁻¹. From these absorption bands shown due to the presence of the ammonium ion, ammonia could be confirmed to be produced and adsorbed to the sulfonic acid group. While ammonia was produced and adsorbed to the sulfonic acid group, the absorption band at 1377 cm⁻¹ was decreased, and the absorption band of ammonium ion was shown. In the presence of O₂, the absorption bands of ammonium ion were observed at 1440 and 1410 cm⁻¹, which were smaller than in the absence of O₂. This means that ammonia was produced through reaction between NO₂ and H₂.

FIG. 6 shows IR spectra of the Cu—Mn catalyst, the Fe—Cr catalyst and the Fe—Mn catalyst, in addition to the Cu—Cr catalyst, in the presence of O₂. As in the Cu—Cr catalyst, the absorption bands of ammonium ion were shown at 1440 and 1410 cm⁻¹ in the presence of O₂, although being small. As such, the degree of production of ammonia greatly varied depending on the kind of component of the catalyst. In the case of the Fe—Cr catalyst and the Cu—Mn catalyst, the absorption band of ammonium ion was very small. In the Fe—Mn catalyst, the absorption band of ammonium ion was large, from which more production of ammonia could be confirmed.

Example 6 H₂-SCR in Flow Reactor

The NO₂ reduction performance of the catalyst using a H₂ reducing agent was measured by use of a normal pressure flow reactor. The construction of the flow reactor used in the H₂-SCR reaction is shown in FIG. 7. 0.1 g of the Pt-supported catalyst was loaded in a quartz tube having an outer diameter of 10 mm and then activated at 500° C. for 1 hour. In consideration of the temperature of diesel exhaust gas, cooling to 150° C. was performed, and a gas mixture of 520 ppm NO₂ by volume and 5% O₂ by volume was supplied at a flow rate of 100 ml/min and thus saturated and adsorbed to the catalyst. While the gas mixture of NO₂ and O₂ was allowed to flow, 2 ml of the H₂ reducing agent was injected five times at intervals of 5 min, thus evaluating the NO₂ reduction performance. Using a NO₂ sensor (NGK, TCNS6005-C3) and a mass spectrometer (Balzer, QMS200) provided to the ends of the reactor, the amount of reduced NO₂ and the amount of consumed H₂ were measured. As such, not only NO₂ but also N₂O and NH₃ can be sensed by use of the NO₂ sensor provided to the end of the reactor, and thus a decrease in the concentration of NO₂ may be connected with conversion of the entirety of NO₂ into N₂. Using the mass spectrometer, it could be seen to produce H₂ at m/e of 2 and water at m/e of 17 and 18. The m/e of NO is 44, and thus the amount of NO₂ converted into NO can be detected. The amount of supported precious metal, partial pressure of O₂, and activity of Pd-supported catalyst were evaluated.

FIG. 8 shows the results of H₂-SCR reaction in the Pt(2.0)-Fe catalyst (a), the Pt(2.0)-Mn catalyst (b) and the Pt(2.0)-Fe—Mn catalyst (c) under conditions of 10% O₂. H₂ was supplied to the NO₂ gas at a predetermined flow rate, thus measuring the NO₂ conversion. The SCR performance by H₂ in the metal oxides alone and in combinations thereof greatly varied. Specifically, in the Pt(2.0)-Fe catalyst, the reduction reaction using H₂ was barely performed at 150˜300° C. Also in the Pt(2.0)-Mn catalyst, the reduction reaction was barely performed even in the presence of H₂ at 150° C., but the NO₂ reduction reaction using H₂ proceeded at 200˜250° C. Because of the injection of H₂, part of the NO₂ was desorbed from the surface of the catalyst and thus the concentration of NO₂ became larger than the initial concentration, but was immediately decreased and thus NO₂ was reduced. However, at 300° C., in lieu of the reduction reaction, the NO₂ desorption became extreme due to the injection of H₂. The reaction results for the Pt(2.0)-Fe—Mn catalyst which is a mixed catalyst of Fe and Mn are shown in (c) of FIG. 8. At 150° C., the NO₂ reduction reaction using H₂ slightly proceeded, but considerably progressed due to the injection of H₂ at 200° C., thus remarkably lowering the concentration of NO₂. At 250˜300° C., part of NO₂ was desorbed because of the injection of H₂ but the reduction reaction considerably proceeded. The reduction performance of the catalyst at 200˜300° C. was evaluated to be superior to the extent that the concentration of NO₂ was close to zero.

TABLE 2 NO₂ Reduction by H₂ in Pt- or Pd-supported Mixed Oxide Catalyst Loaded O₂ Removal NO₂ H₂ Amount Content Temp. Limit. Convers. Effi. Catalyst (g) (%) (° C.) (ppm) (%) (%) Pt(0.2)—Fe—Mn 0.1  5^(a)) 150 371 8 1 200 231 18 2 250 317 14 2 300 188 42 5 Pt(2.0)—Fe—Mn ″ ″ 150 385 17 2 200 34 124 16 250 21 128 17 300 30 89 12 Pt(2.0)—Cu—Mn ″ ″ 150 386 8 1 200 374 2 1 250 385 15 2 300 394 17 2 Pt(2.0)—Cu—Mo ″ ″ 150 384 10 1 200 385 14 2 250 379 14 2 300 364 16 2 Pd(2.0)—Fe—Mn ″ ″ 150 355 7 1 200 32 129 17 250 43 112 15 300 72 87 12 Pd(2.0)—Cu—Cr ″ ″ 150 374 14 2 200 188 34 5 250 159 72 10 300 248 21 3 Pt(2.0)—Fe ″ 10^(b)) 150 470 7 1 200 475 3 1 250 463 11 1 300 475 1 0 Pt(2.0)—Mn ″ ″ 150 299 7 1 200 58 60 8 250 68 59 8 300 146 39 5 Pt(2.0)—Fe—Mn ″ ″ 150 379 8 1 200 53 108 14 250 26 109 14 300 33 79 10 Note: NO₂ gas: ^(a))NO₂ 505 ppm/O₂ 5%/N₂ balance ^(b))NO₂ 524 ppm/O₂ 10%/N₂ balance

As seen in FIG. 8 showing the NO₂ reduction using H₂, removal limitation, NO₂ conversion and H₂ efficiency were calculated. The removal efficiency represents the minimum concentration of NO₂ lowered due to the injection of H₂. The NO₂ conversion and the H₂ efficiency are defined as Equations 1 and 2 below.

$\begin{matrix} {\frac{{number}\mspace{14mu} {of}\mspace{14mu} {moles}\mspace{14mu} {of}\mspace{14mu} {NO}_{2}\mspace{14mu} {reduced}}{{number}\mspace{14mu} {of}\mspace{14mu} {moles}\mspace{14mu} {of}\mspace{14mu} {NO}_{2}\mspace{14mu} {fed}\mspace{14mu} {for}\mspace{14mu} 25\mspace{14mu} \min} \times 100} & {{Equation}\mspace{14mu} 1} \\ {\frac{{number}\mspace{14mu} {of}\mspace{14mu} {moles}\mspace{14mu} {of}\mspace{14mu} {NO}_{2}\mspace{14mu} {reduced}}{{number}\mspace{14mu} {of}\mspace{14mu} {moles}\mspace{14mu} {of}\mspace{14mu} H_{2}\mspace{14mu} {injected}} \times 100} & {{Equation}\mspace{14mu} 2} \end{matrix}$

As above, the results of NO₂ reduction by injecting H₂ to the flow of NO₂ in the presence of 5% O₂ and 10% O₂ by volume are shown in Table 2. As is apparent from these results, the NO₂ reduction performance could be seen to greatly vary depending on the amount of supported precious metal, the kind of precious metal, and the component of mixed oxide. When the O₂ content was 5%, the Pt(0.2)-Fe—Mn catalyst in which Pt was supported in an amount of 0.2% by weight exhibited the NO₂ conversion of 8˜42% at 150˜300° C., which was not so high. However, the Pt(2.0)-Fe—Mn catalyst in which the amount of supported Pt was 2.0% had the NO₂ conversion of 17% at 150° C. but exhibited the NO₂ conversion exceeding 100% at 200° C. and 250° C. When the amount of supported Pt was not 0.2% but 2.0%, the NO₂ reduction performance was superior. The effect of increasing the NO₂ conversion exceeding 100% is caused by reducing the fed NO₂ by produced ammonia, adsorbing the remaining ammonia to the surface of the catalyst or removing lattice O from the surface of the catalyst to thus additionally remove NO₂. Because ammonia is produced from NO₂ and is used to remove NO₂, the point of time at which H₂ is supplied does not agree with the point of time at which NO₂ is removed.

In the Pt(2.0)-Cu—Mn catalyst and the Pt(2.0)-Cu—Mo catalyst, having different components, the NO₂ conversion was very low to the level of 2˜17%. On the other hand, the Pd(2.0)-Fe—Mn catalyst in which Pd was supported in place of Pt had the reduction performance similar to that of the Pt(2.0)-Fe—Mn catalyst, and thus exhibited the NO₂ conversion of 129% at 200° C. which was evaluated to be very high. The Pd(2.0)-Cu—Cr catalyst exhibited the NO₂ conversion of 72% at 250° C., which was smaller than that of the Pd(2.0)-Fe—Mn catalyst but was evaluated to be high. As shown in FIG. 7, for a considerably long period of time after completion of the injection of H₂, the phenomenon in which the concentration of NO₂ is lowered, briefly, the reduction reaction, slowly proceeds. This is because the produced ammonia reduces the surface of the catalyst to thus produce NO_(x) adsorption sites or is adsorbed unchanged and then reacted with fed NO_(x).

The mixed oxide catalyst, for example, the Pt(2.0)-Fe—Mn catalyst, exhibiting excellent performance in the presence of 5% O₂, was evaluated for reduction performance under an excessive O₂ content of 10%. In the Pt(2.0)-Fe catalyst, the NO₂ conversion was very low on the order of 10% or less. In the Pt(2.0)-Mn catalyst, the NO₂ conversion was considerably high on the order of 60%. In the Pt(2.0)-Fe—Mn catalyst, the NO₂ conversion exceeded 100% and the removal limitation was very low on the order of 26 ppm. Even under an excessive O₂ content of 10%, the NO₂ reduction performance was superior.

Example 7 H₂-SCR in Reactor Packed with Precious Metal-Supported Mixed Oxide Catalyst and NH₃-SCR Catalyst

The reactor was packed with the Pt(2.0)-Fe—Mn catalyst having superior NO₂ reduction performance by H₂ in Table 2 and the NH₃-SCR catalyst prepared in Example 1, for example, the Fe-BEA zeolite, and NO₂ reduction performance by H₂ was evaluated. As shown in FIG. 9, almost all of NO₂ was removed at 200˜300° C. due to injection of H₂. After completion of the injection of H₂, the concentration of NO₂ was maintained low for a considerably long period of time, from which the reduction performance was evaluated to be high. The reaction results are shown in Table 3 below. At 200° C., the NO₂ conversion was 133% which was evaluated to be very high.

TABLE 3 NO₂ Reduction by H₂ in Reactor packed with Fe-BEA Zeolite and Mixed Oxide Catalyst O₂ Re- Con- moval NO₂ H₂ Loaded tent Temp. Limit. Convers. Effi. Catalyst Amount (%) (° C.) (ppm) (%) (%) Pt(2.0)—Fe—Mn 0.1 g 10 150 379 8 1 200 53 108 14 250 26 109 14 300 33 79 10 Pt(2.0)—Fe—Mn + Each 0.1 g, ″ 150 372 10 1 Fe-BEA Total 0.2 g 200 24 133 16 250 26 111 14 300 33 91 11 Note: NO₂ gas: NO₂ 505 ppm/O₂ 10%/N₂ balance

Example 8 Hydrothermal Treatment of Mixed Oxide Catalyst

In diesel exhaust gas containing a considerable amount of water, when the temperature of exhaust gas is widely changed depending on driving conditions, the catalyst should be used without exchange for a long period of time and thus should have high hydrothermal stability. Thus, the hydrothermal stability of the mixed oxide catalyst was evaluated.

The precious metal-supported mixed oxide catalyst was loaded in an alumina crucible, and was placed in a quartz tube of a circular burning furnace and thus subjected to hydrothermal treatment. Then, N₂ was allowed to flow into a steam evaporator in a precision constant temperature circulator, thus preparing and supplying a gas mixture containing N₂ and 10% steam by volume. While the N₂ gas containing steam was supplied at a flow rate of 100 ml/min, the treatment was performed at 750° C. for 4 hours.

The catalyst was washed with water and the decrease in the activity of the mixed oxide catalyst was evaluated. 10 g of the Pt(2.0)-Fe—Mn catalyst as the Pt-supported catalyst was added to 1000 g of water, strongly stirred at room temperature, treated for 1 hour, filtered using filter paper and then dried in an oven at 80° C.

TABLE 4 Amount of NO₂ Stored on Pt-supported Mixed Oxide Catalyst after Waste Treatment & Hydrothermal Treatment NO₂ Adsorption Amount (mg/g) Adsorption Storage Catalyst Amount Amount Pt (2.0)—Fe—Mn 182 181 Pt (2.0)—Fe—Mn (Water Treatment) 72 66 Pt (2.0)—Fe—Mn (Hydrothermal Treatment) 84 82

The amount of NO₂ stored on the Pt(2.0)-Fe—Mn catalyst subjected to water treatment and hydrothermal treatment is shown in Table 4. The amount of stored NO₂ through water treatment and hydrothermal treatment was lowered from 181 mg/g to 66 mg/g and 82 mg/g respectively. The amount of stored NO₂ was considerably lowered through hydrothermal treatment.

Table 5 below shows the results of NO₂ reduction by H₂ in the Pt-supported Fe—Mn catalyst subjected to water treatment and hydrothermal treatment. The reduction performance of the Pt(2.0)-Fe—Mn catalyst subjected to water treatment was similar to that before the treatment. The removal performance was slightly decreased at high temperatures, but was greatly increased at low temperatures. In the conventional Pt(2.0)-Fe—Mn catalyst, the NO₂ conversion and the H₂ efficiency at 150° C. were 17% and 2% respectively, and thus the removal performance was poor. In the water treated catalyst, the NO₂ conversion and the H₂ efficiency were considerably increased to 73% and 10% respectively, but were slightly decreased at 250 ˜300° C.

In the Pt(2.0)-Fe—Mn catalyst subjected to hydrothermal treatment, the NO₂ reduction performance at 150˜200° C. was considerably deteriorated, and was greatly improved at temperatures not lower than 250° C. The Pt(2.0)-Fe—Mn catalyst exhibited had the NO₂ conversion and the H₂ efficiency of 89% and 12% respectively at 300° C., and the NO₂ conversion and the H₂ efficiency of the Pt(2.0)-Fe—Mn catalyst subjected to hydrothermal treatment were considerably improved to 121% and 16% respectively.

TABLE 5 NO₂ Reduction by H₂ in Pt(2.0)—Fe—Mn Catalyst Loaded O₂ Removal NO₂ H₂ Amount Content Temp. Limit. Convers. Effi. Catalyst (g) (%) (° C.) (ppm) (%) (%) Pt(2.0)—Fe—Mn 0.1 5 150 385 17 2 200 34 124 16 250 21 128 17 300 30 89 12 Pt(2.0)—Fe—Mn ″ ″ 150 66 73 10 (Water Treatment) 200 39 114 15 250 26 107 14 300 50 81 11 Pt(2.0)—Fe—Mn ″ ″ 150 450 2 1 (Hydrothermal 200 147 28 4 Treatment) 250 86 84 11 300 39 121 16

Example 9 Durability of Precious Metal-Supported Mixed Oxide Catalyst to Sulfur Poisoning

The diesel exhaust gas contains sulfur compounds including SO₂, undesirably deteriorating the activity of the catalyst.

To evaluate the durability of the precious metal-supported mixed oxide catalyst to sulfur poisoning, the catalyst was poisoned with SO₂. The catalyst was exhausted, activated and then sufficiently poisoned with SO₂ gas at 10 Torr at 150° C. for 1 hour, before storing NO₂. After exhaust for 1 hour, NO₂ at 30 Torr was supplied and thus the adsorption amount thereof was measured.

The amount of adsorbed SO₂ and the amount of stored NO₂ after poisoning are shown in Table 6 below. At 150° C., SO₂ was not adsorbed in a large amount. Upon treatment with SO₂, the amount of stored NO₂ was decreased by almost half.

TABLE 6 Amounts of SO₂ and NO₂ Stored on Pt (2.0)—Fe—Mn catalyst SO₂ Adsorption NO₂ Adsorption Amount (mg/g) Amount (mg/g) Before After Before After Catalyst Exhaust Exhaust Exhaust Exhaust Pt (2.0)—Fe—Mn — — 182 181 Pt (2.0)—Fe—Mn 13 13  68  64 (Poisoned)

The deterioration in the reduction performance of the precious metal-supported mixed oxide through sulfur poisoning at 150° C. and 200° C. was evaluated. 2 ml of SO₂ gas amounting to 8 times that of NO₂ supplied for 5 min after NO₂ saturation and adsorption was injected, thus poisoning the catalyst. Subsequently, H₂ was injected and thus the deterioration in the activity of the catalyst was measured.

The NO₂ removal performance by H₂ after sulfur poisoning is shown in Table 7 below. The NO₂ reduction performance of the Pt(2.0)-Fe—Mn catalyst poisoned with SO₂ was almost the same as that before sulfur poisoning. The NO₂ conversion and the H₂ efficiency were slightly decreased, but the NO₂ removal limitation at 200° C. was 35 ppm which was equivalent to that before sulfur poisoning.

TABLE 7 NO₂ Reduction by H₂ in Pt(2.0)—Fe—Mn Catalyst poisoned with Sulfur Loaded O₂ Removal NO₂ H₂ Amount Content Temp. Limit. Convers. Effi. Catalyst (g) (%) (° C.) (ppm) (%) (%) Pt(2.0)—Fe—Mn 0.1 5 150 385 17 1 200 34 124 16 Pt(2.0)—Fe—Mn ″ ″ 150 285 15 2 (Poisoned) 200 35 108 14 

1. A mixed oxide catalyst for reducing NO_(x) using H₂ as a reducing agent, comprising one or more A metal oxides selected from the group of consisting of Fe₂O₃, CO₂O₃, NiO and CuO; and one or more B metal oxides selected from the group consisting of V₂O₅, Cr₂O₃, MnO₂ and MoO₃, which are co-precipitated and mixed.
 2. The mixed oxide catalyst according to claim 1, wherein a ratio of A metal oxide to B metal oxide ranges from 2:1 to 1:0.5.
 3. The mixed oxide catalyst according to claim 1, wherein a precious metal selected from the group consisting of platinum and palladium is supported in an amount of from 0.1 to 2 wt %.
 4. The mixed oxide catalyst according to claim 3, further comprising an ammonia-selective catalytic reduction catalyst.
 5. A method of preparing a mixed oxide catalyst for reducing NO_(x) using H₂ as a reducing agent, comprising: dissolving in aqueous nitrate or acetate one or more metal oxide precursors of A metal oxides, wherein the A metal oxides are selected from the group consisting of Fe₂O₃, CO₂O₃, NiO and CuO, and one or more metal oxide precursors of B metal oxides, wherein the B metal oxides are selected from the group consisting of V₂O₅, Cr₂O₃, MnO₂ and MoO₃, thus preparing a solution; adding barium nitrate oxide to the solution; adding ammonia water or aqueous sodium bicarbonate as a precipitating agent to the solution so that pH of the solution is from 7 to 7.3, thus forming a precipitate; and subjecting the precipitate to post treatment comprising filtering, washing, drying and burning.
 6. The method according to claim 5, wherein a ratio of A metal oxide to B metal oxide ranges from 2:1 to 1:0.5.
 7. The method according to claim 5, further comprising, after subjecting the precipitate to post treatment, supporting either or both precious metals of platinum and palladium in an amount of from 0.1 to 2 wt % on the catalyst, burning the precious metal-supported catalyst, and subjecting the burned catalyst to reduction treatment using a gas mixture containing N₂ and H₂ at a molar ratio of
 1. 8. A method of reducing NO_(x) at from 150 to 350° C. using H₂ as a reducing agent in presence of the mixed oxide catalyst of claim
 1. 9. The method according to claim 8, wherein the method is performed under conditions in which O₂ content is equal to or less than 10% by volume based on a volume of the NO_(x). 